Piezoelectric ceramic materials, based on lead-ziconate-titanate (pzt), comprising valence-compensated complexes containing ag

ABSTRACT

The characteristics of piezoceramic multilayer actuators based on lead-zirconate-titanate are determined to a great extent by the compatibility of PZT ceramics having a low sintering temperature with the AgPd internal metallisation during cofiring. It is important to take into consideration that Ag ions in PZT modifications have a high diffusivity at high temperatures (&gt;800° C.) and in addition act as acceptor doping when integrated into the PZT system. The reduction of the fraction of the precious metal palladium, which prevents diffusion, is limited, as silver increasingly diffuses into the piezoceramic as the silver fraction in the internal electrodes increases. According to the invention, Ag +  ions are used to form valence-compensated compositions of the PZT system. A higher level of deformation is maintained, i.e. the acceptor-donor effect in the system is very similar to that of the PZT system modified conventionally without internal electrodes.

[0001] The invention relates to the modification of piezoelectricceramic materials based on lead zirconate titanate (PZT).

[0002] The properties of piezoceramic multi-layer actuators based onlead zirconate titanate are determined to a great extent by thecompatibility of low-sintering PZT-ceramic with the AgPd internalmetalization during co-firing.

[0003] It should be noted here that Ag ions in PZT modifications havehigh diffusivity at high temperatures (>800° C.) and also act as anacceptor dopant when incorporated into the PZT system. A non-, or onlypartly, compensated acceptor substitution by donor ions such as Nb⁵⁺ canlead to drastic changes in the ferroelectric properties and thus also inthe deformation behavior of actuator materials. This effect must betaken into account when selecting the AgPd alloy for the internalelectrodes. There is a limit to how far the content of the noble metalpalladium can be reduced, as an increase in the silver content of theinternal electrodes leads to increased diffusion of the silver in thepiezoceramic.

[0004] The use of the alloy AgPd 70/30 for the internal metalizationfulfills the premises for compatibility with the piezoceramic in thecofiring process. A Pd content of ≧30% has the effect of preventing thediffusion of silver.

[0005] Ag-additives to originally valence-compensated PZT modificationsact as acceptor dopants, which influence the polarity behavior. Thisalso results in a reduction in deformation in the vicinity of anelectric field.

[0006] Accordingly, the deformation characteristic is not stabilized byanticipating the introduction of Ag⁺ ions when sintering piezoceramiclaminates with AgPd internal metalization by adding Ag when formulatingthe composition of conventionally modified PZT compositions.

[0007] The object of the invention is to prevent diffusion of Ag in theceramic material as far as possible in spite of a reduced Pd content ofthe electrode material.

[0008] The object is achieved with the aid of the characterizingfeatures of claims 1 and 2. Advantageous developments of the inventionare claimed in the subordinate claims.

[0009] The invention consists in that in the modifications ofpiezoelectric ceramic materials based on lead zirconate titanate asvalence-compensated compositions with n complex compounds in themulti-component systems with the general formulaPbTiO₃—PbZrO₃-Σ_(n)AM′_(α)M″_(β)O₃ with heterovalent ions, at least oneion M″ is represented by Ag⁺ and M′ are pentavalent cations of Sb, Ta,Nb, wherein n≦3, α=¼, β=¾, the content of the respective complexcompounds being ≦20 mol % and A≡A²⁺, populated by ions of the elementsPb, Ba, Ca, Sr.

[0010] The multi-component systems with the general formulaPbTiO₃—PbZrO₃-Σ_(n)A′_(α)A″_(β)BO₃ with heterovalent ions are modifiedso that at least one ion A″ is represented by Ag⁺ and A′ are trivalentcations of Bi, La, Nd, wherein n≦3, α=β=, the content of the respectivecomplex compounds being ≦20 mol % and B≡B⁴⁺, populated by ions of theelements Ti, Zr.

[0011] In the modified system, the hypothetical complex Sr(Ag_(0.25)Nb_(0.75))O₃ in a PZT-SAN(1-y)Pb(Zr_(x)Ti_(1-x))O₃-ySr(Ag_(0.25)Nb_(0.75))O₃ system, for example,may be considered as a coupled substitution of Ag⁺ acceptor ions andNb⁵⁺ donor ions.

[0012] The composition0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O₃ of the PZT-SANsystem with an Ag content of 0.178 wt. % was compared with the modelsubstance with the non-stoichiometric composition0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.00)Nb_(0.75))O₃ without Ag⁺ions for acceptor-donor valence compensation. The properties show thatit is not possible to achieve valence compensation by removing theacceptor deficit via the diffusion of Ag from the internal electrodes.The deformation of the non-stoichiometric composition is substantiallyless than that of the stoichiometric composition.

[0013] The invention is explained in more detail with the aid of anembodiment. The polarization and deformation behavior of laminates withand without internal electrodes was investigated. The interaction of themodified PZT ceramics with different AgPd alloys (AgPd 70/30 and AgPd85/15) during cofiring of the laminates is assessed and a solution forthe reduction of the Pd content <30 is indicated.

[0014] Knowledge of effective x- and y-concentrations arises from thedevelopment of the PZT system: The properties relevant for theapplication of the modified PZT ceramics for sensors and actuators canbe optimized in a narrow concentration range of x=0.46 to 0.49 fory=0.02. Accordingly, in an example, a PZT-SAN composition series 0.98Pb(Zr_(x)Ti_(1-x))O₃-0.02 Sr(Ag_(0.25)Nb_(0.75))O₃ for x=0.51 to 0.55was formulated in steps from Δx=0.005 and synthesized by the mixed oxidemethod.

[0015] When synthesizing (sintering) the PZT-SAN ceramics, an increasedaction of Ag⁺ ions as acceptor dopants with coupled substitution of Nb⁵⁺donor ions was already

[0016] The Characteristic Functional Properties (Piezoelectric andDielectric Coefficients) of the Compositions

[0017] 0.98Pb(Zr_(0.52)Ti_(0.48)) 03-0.02Sr(Ag_(0.25)Nb_(0.75)) 03 and0.98Pb(Zr_(0.53)Ti_(0.47)) 03-0.02Sr(Ag_(0.25)Nb_(0.75))O₃ from thisseries of compositions are given in the following table as examples. x =0.52 x = 0.53 Curie temperature T_(c), ° C. 350 348 Coercive fieldstrength E_(c), 10⁶V/m 1.27 1.11 Permittivity ε₃₃ ^(T)/ε₀ 1959 2077Dielectric loss factor tan δx 10⁻⁴ 129 145 Planar electromechanicalcoupling k_(p) 0.62 0.66 factor Piezo modulus d₃₃, pC/m 423 498Re-orientation S₃, 0/00 2.09 2.16 deformation (E = 3 · 10⁶V/m)

[0018] Dielectric and Electromechanical Properties of PZT-SAN Ceramics

[0019] When forming valence-compensated compositions of the PZT-SANsystem, a high level of deformation is obtained upon polarization.

[0020] The influence of the diffusion of silver during cofiring of thelaminates with internal electrodes of differing Pd contents is lesspronounced. The values of the coercive field strength and deformationfor the composition0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O₃ of the PZT-SANsystem for example are virtually the same for both internalmetalizations, as can be seen from FIGS. 1 and 2.

[0021]FIG. 1 shows the polarization behavior and

[0022]FIG. 2 shows the deformation behavior of sintered laminates of0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))O₃ ceramics withand without AgPd internal electrodes.

[0023]FIG. 2 shows the deformation behavior of sintered laminates of(Zr_(0.52)Ti_(0.48))O_(30.0.02)Sr(Ag_(0.25)Nb_(0.75))O₃ ceramics withand without AgPd internal electrodes.

[0024] Reducing the diffusion of silver during cofiring of laminateswith internal electrodes makes it possible to use AgPd alloys withreduced Pd content. The values of coercive field strength anddeformation for the composition of0.98Pb(Zr_(0.52)Ti_(0.48))O₃-0.02Sr(Ag_(0.25)Nb_(0.75))03 of the PZT-SANsystem for internal metalization with AgPd 70/30 and AgPd 85/15 arevirtually the same. When using Ag⁺ ions to form valence-compensatedcompositions of the PZT system, a high level of deformation is stillobtained, i.e. the acceptor-donor action of the system is very similarto that of the conventionally modified PZT system without internalelectrodes, as can be seen from FIG. 2.

1-7 (Canceled)
 8. A modification of a piezoelectric ceramic materialbased on lead zirconate titanate as a valence-compensated compositionwith an n complex compound in the multi-component system having thegeneral formula PbTiO₃—PbZrO₃Σ_(n)AM′_(α)M″_(β)O₃ with heterovalentions, wherein at least one ion M″ is represented by Ag⁺ and M′ arepentavalent cations of Sb, Ta, Nb, wherein n≦3, α=¼, β=¾, the content ofthe respective complex compounds being ≦20 mol. %, and A≡A²⁺ populatedby ions of the elements Pb, Ba, Ca, Sr.
 9. A modification of apiezoelectric ceramic material based on lead zirconate titanate as avalence-compensated composition with an n complex compound in themulti-component systems with the general formulaPbTiO₃—PbZrO₃-Σ_(n)A′_(α)A″BO₃ with heterovalent ions, wherein at leastone ion A″ is Ag⁺ and A′ is a trivalent cation selected from the groupconsisting of Bi, La and Nd, wherein n≦3, α=β=½, the content of therespective complex compounds being ≦20 mol. % and B≡B⁴⁺, and is an ionof an element selected from the group consisting of Ti and Zr.
 10. Amodification of a piezoelectric ceramic material based on lead zirconatetitanate according to claim 8, wherein the Pb ion in the base systemPbTiO₃-PbZrO₃(PZT) is substituted partially, by up to 10 mol %, with atleast one divalent ion selected from the group consisting of Ba, Ca andSr.
 11. A modification of a piezoelectric ceramic material based on leadzirconate titanate according to claim 9, wherein the Pb ion in the basesystem PbTiO₃-PbZrO₃(PZT) is substituted partially, by up to 10 mol %,with a divalent ion of an element selected from the group consisting ofBa, Ca and Sr.
 12. A modification of a piezoelectric ceramic materialbased on lead zirconate titanate of the formula(1−y)Pb(Zr_(x)Ti_(1-x))O₃-yA²⁺(Ag_(α)B_(β) ⁵⁺)O₃, wherein A²⁺ is an ionof an element selected from the group consisting of Pb, Ba, Ca and Srand B⁵⁺ is an ion selected from the group consisting of an element Sb,Ta and Nb and y=0.001 to 0.10, x=0.45 to 0.55 and α=¼ and β=¾.